1. Field of the Invention
The present invention relates generally to flexible endoscopes; specifically to an improvement in the design of their flexible insertion tubes.
2. State of the Prior Art
A typical endoscope comprises an elongated tube suitable for introduction into a human or animal body. A lens at a distal tip of the endoscope forms an image of an internal area of the body. Means such as fiber optic cables or video transmission transmit the image along the length of the endoscope to a point outside of the body where it can be viewed by a surgeon or other user of the endoscope. Of course, endoscopes are not limited to medical uses, and are also useful in machine diagnostics and repair among other uses. Regardless of its intended use, a flexible distal portion allows the endoscope to negotiate non-linear passages.
In addition to the image transmission means, the distal portion of the endoscope typically contains one or more tubular passages for passing air, liquid, or instruments. Flexible endoscopes usually also contain one or more wires for controlling movement of the tip of the endoscope. A flexible sheath surrounds the flexible portion of the endoscope to protect it from its environment and to protect a patient's body or other environment from the internal parts of the endoscope.
To prevent noscomical infection, endoscopes are typically washed and then either sterilized or subjected to high level disinfection after each use. For added convenience during these procedures, many endoscopes are provided with an entirely water tight structure to protect the internal components of the endoscope from washing, disinfection and sterilization agents. In a flexible endoscope, the flexible elastomeric sheath surrounding the flexible portion of the endoscope forms an integral part of this water tight structure.
Delicate medical instruments, such as flexible endoscopes and the like, are notoriously difficult to sterilize and disinfect due to the complexity of their structure and design. Elastomeric parts on flexible endoscopes cannot survive the intense heat of steam sterilization typically used in the hospital and clinical environment. Typically, these instruments are now dipped into baths of liquid sterilants or high level disinfectants, with some of the liquid being forced through the long lumens within the endoscopes. Such processes have limitations. For instance, the high toxicity of many of the preferred liquid sterilants or disinfectants classifies them as hazardous waste after the procedure and makes them dangerous to work with. Also, liquid does not penetrate small crevices within an instrument as well as gaseous phase sterilants such as high pressure steam and gaseous chemical sterilants.
Gaseous sterilization with strong oxidizing agents such as hydrogen peroxide is a well established method for sterilizing delicate instruments such as flexible endoscopes. Ethylene oxide (EtO) gas is one such sterilant. However, it must be handled carefully as it is extremely toxic and mutagenic. One particularly effective gaseous technology is hydrogen peroxide gas plasma sterilization such as that provided by the STERRAD.RTM. Systems of Advanced Sterilization Products, a division of Johnson & Johnson Medical, Inc. In this type of system, instruments are placed into a sealed chamber and exposed to an atmosphere containing hydrogen peroxide in the gaseous phase. The chamber is placed under a vacuum prior to admitting the hydrogen peroxide to encourage the hydrogen peroxide vapor to reach all areas of the instrument. Once the vapor has reached all surfaces on the instruments in the chamber, an electromagnetic field is applied to the chamber driving the hydrogen peroxide into the plasma phase of matter. This enhances the sterilizing effect of the hydrogen peroxide. Further, when the field is released, the free radicals in the plasma recombine to form water and oxygen, thereby leaving no harmful residuals.
However, when flexible endoscopes have been subjected to this type of process, many experienced rapid degradation of their elastomeric outer sheath. This was curious as it was not thought that the hydrogen peroxide would affect such parts. Even more perplexing was the apparent random nature of the problem. Many theories were propounded, including some unknown interaction between the hydrogen peroxide, the plasma state and the elastomers. It was discovered that the degradation stems not from the action of the oxidizer on the elastomer, but from the action of the oxidizer on lubricating substances within the interior space within the insertion tube which in turn form compounds which degrade the elastomers. Certain lubricants found in endoscopes and other instruments breakdown in the oxidizing environment of the hydrogen peroxide vapor to form acids which can damage the elastomeric parts of delicate medical instruments. The lubricants are members of the class of metal dichalcogenides, such as molybdenum disulfide.
The oxidative chemical sterilant vapor reaches interior space primarily through two avenues. First, vapor may enter the space through a pressure relief port in the insertion tube. A significant area of unused space occupies the interior of the insertion tube of most flexible endoscopes. Of course, this space is filled with gas, typically air. As the pressure is reduced during a sterilization procedure, the gas trapped inside of the endoscope exerts tremendous pressure against the elastomeric sheath. If this pressure is not released, the sheath could rupture. Many endoscopes are provided with a sealable port leading into the interior of the endoscope. During sterilization in a reduced pressure environment the port may be opened to allow the interior of the endoscope to communicate with the sterilization atmosphere and thus relieve the excess pressure within the endoscope. The port is also used to check for leaks in the endoscope, especially in the sheath, through the controlled application of gas pressure to the endoscope's interior while it is submerged in water. Co-pending U.S. patent application Ser. No. 08/446,377, and its foreign equivalents including EPO Application No. 96303585.2, incorporated herein by reference, disclose a two-way check valve and filter mechanism to relieve the pressure differential across the outer sheath without admitting hydrogen peroxide or other gaseous sterilants into the inner space of the insertion tube.
Second, polyurethane is a preferred elastomer for forming the outer sheath due to its biocompatibility, surface lubricity softness and to the ease with which it can be manufactured. However, oxidative sterilant vapors such as hydrogen peroxide can permeate through a sheath formed of polyurethane to enter the interior space of the endoscope. Once within the interior space it interacts with lubricants such as molybdenum disulfide to form acidic products which attack the polyurethane sheath. This action also degrades the effectiveness of the lubricant, but the sheath generally fails before the lubricant is exhausted in this manner. Further, the chemical sterilant vapor may cause other damage within the insertion tube and it may not dissipate during the normal course of the sterilization cycle, leaving residual chemical sterilant within the interior space after the sterilization procedure is completed, thus extending the chemical reaction between sterilants gas and molybdenum disulfide.
U.S. Pat. No. 4,753,222, incorporated herein by reference, discloses an endoscope sheath with a two layer construction. However, it does not disclose that the inner layer may be made to prevent passage of hydrogen peroxide or other sterilants into the interior space of the insertion tube.